1. Field of the Invention
The present invention relates to acidic treatment fluids used in subterranean operations, and more specifically, to acidic treatment fluids including citric acid and salts of ethylene di-amine tetra-acetic (EDTA), and methods of use in subterranean operations.
2. Description of the Related Art
It is a common practice to acidize subterranean formations in order to increase the permeability thereof. For example, in the petroleum industry, it is conventional to inject an acidizing fluid into a well in order to increase the permeability of a surrounding hydrocarbon-bearing formation, thereby facilitating the flow of hydrocarbons into the well from the formation. Such acidizing techniques are generally referred to as “matrix acidizing” treatments.
In matrix acidizing, the acidizing fluid is passed into the formation from the well at a pressure below the breakdown pressure of the formation. In this case, increase in permeability is affected primarily by the chemical reaction of the acid within the formation with little or no permeability increase being due to mechanical disruptions within the formation as in fracturing.
In acidizing treatments, various acid solutions are used to dissolve deposits of calcium compounds in order to increase the permeability of subterranean well formations. For example, hydrochloric acid (HCl), acetic acid, formic acid, and different mixtures of these acids are commonly used. Although hydrochloric acid is the most common used acid, acetic and formic acids are excellent alternatives to HCl in deep hot formations due to their retarded reaction rates.
Citric acid (C6H8O7), which is an α-hydroxy carboxylic acid with three carboxylic (—COOH), and one hydroxyl (—OH) groups, has been sparingly used as an alternative to common weak acids in acidizing treatments of deep hot reservoirs. Citric acid has been used in methods of acidizing a reservoir while steadily sequestering iron in order to prevent precipitation of different iron scales, such iron sulfide and iron oxide. Compared to HCl acid systems, citric acid is less corrosive to tubulars and can be inhibited by the use of inexpensive corrosion inhibitors, resulting in a cost saving per treatment. One of the main obstacles involved with the use of citric acid, however, is the re-precipitation of calcium ions as calcium citrate, which is the reaction product that has a low solubility in water, when the pH value of reaction solution exceeds 3. This precipitation can reduce the permeability of the treated formation.
Besides calcium citrate, during acidizing treatments, various compounds, such as iron and other metallic compounds, tend to precipitate in the spent acidic treatment fluids. Various types of sequestering additives have been used to alleviate this problem. None of the known additives, however, have been successful at preventing precipitation of calcium citrate during acidizing treatments utilizing citric acid.
A need exists for effective sequestering additives that are capable of simultaneously preventing precipitation of both iron-based compounds and calcium citrate during acidizing treatments. It would be helpful if such additives were effective when used with citric acid based acidizing treatment fluids.